The present invention relates to galvanomagnetic position sensing of a rotating shaft.
It is well known in the art that the voltage modulation of Hall elements or the resistance modulation of magnetoresistors can be employed in position and speed sensors with respect to moving magnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object moving relative, and in close proximity, to the MR, such as a tooth, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth is adjacent to the MR than when a slot is adjacent to the MR.
Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position information during cranking. Various combinations of magnetoresistors and single and dual track toothed or slotted wheels (also known as encoder wheels and target wheels) have been used to obtain this information (see for example U.S. Pat. Nos. 5,570,016, 5,714,883, 5,731,702, and 5,754,042).
The crank position information is encoded on a rotating target wheel in the form of teeth and slots. Virtually all such sensors are of the magnetic type, either variable reluctance or galvanomagnetic (e.g. Hall generators or magnetoresistors). Galvanomagnetic sensors are becoming progressively most preferred due to their capability of greater encoding flexibility and speed independent output signals.
Furthermore, virtually all brushless motors require rotor position sensors to operate. The simplest and most common method of sensing rotor position is by means of an arrangement of three individual Hall sensors. These sensors have to be placed at specific locations on the motor stator periphery, and spaced 120 degrees from each other. In addition, they provide only a very basic commutation signal. If more precise motor control and operation are required, then an additional high-resolution sensor is also needed.
Siemens Corporation disclosed in a 1982/83 data book a noncontact differential MR sensor for detecting shaft rotation position. In this disclosure, a pair of rectangularly shaped and parallel arranged MRs are located adjacent to, and axially aligned with, the end of a rotating shaft, wherein the shaft has a rectangular permanent magnet mounted thereto which covers a portion of the end of the shaft. With 360 degrees of rotation of the shaft, the permanent magnet controls the MR sensor (part FP212L100) to generate a single sine-like output signal. This output signal has low accuracy due to the limited range of angular position resolution that a single sine curve can provide. This low accuracy limits the usefulness of this sensor for motor control applications.
Therefore, what remains needed is a simple and inexpensive noncontact sensor which provides high resolution rotor position at all times.
The present invention provides a galvanomagnetic position sensor, wherein a single die MR sensor senses the rotation of the end face of a motor shaft having a magnetic irregularity (as for example a semicircular ferromagnetic tooth) and generates at least two periodically varying voltage signals having a predetermined phase spacing therebetween (as for example three sinusoidal signals with 120 degree phase shifts). In addition, disclosed is a unique MR die configuration for this sensor, and the appropriate sensor interface circuit therefor.
In a first implementation of the present invention, the single MR sensor consists of an MR die incorporating a plurality of matched pie-shaped MR elements collectively forming a circular area. Structurally, the MR die consists of a plurality of electrically independent pie shaped segments, each characterized by a magnetosensitive material. In the first implementation of the present invention, the segments are by way of example composed of indium antimonide (InSb) epitaxial film mesas, or film mesas of another suitable magnetosensitive material. The techniques to fabricate the MR elements is elaborated in U.S. Pat. No. 5,153,557, issued Oct. 6, 1992 and U.S. Pat. No. 5,184,106, issued Feb. 2, 1993, both of which being hereby incorporated by reference herein.
It is preferable that the MR elements be matched to each other and that the geometry of the MR elements be such that the magnitude of the increase of the resistance of one MR element is the same as the magnitude of the decrease in resistance of a diametrically opposed MR element, but this is not essential. Proper circuit design with appropriate weighting factors, determined empirically or theoretically, can be applied by those of ordinary skill in the art to accommodate MR element mismatch and geometries.
The MR die is mounted on a bias magnet and affixed to face toward the end face of the motor shaft which is made of a ferromagnetic material. The center of the shaft is aligned with the center of the MR die and the end face of the shaft is parallel to the plane of the MR die. One half of the shaft end face is a few millimeters shorter than the other half creating a half circular tooth covering one half of the total MR die area. When the motor shaft rotates, that tooth sweeps past the MR elements and covers one half of the total MR die area at all times. Due to the higher magnetic flux density under the tooth, the MR elements under the tooth increase their resistance resulting in the resistance modulation of each MR element between a maximum resistance and a minimum resistance. By a proper selection of the MR elements, a properly designed circuit can obtain three sinusoidal voltage signals spaced 120 degrees apart. For example, the resistance changes of the MR elements can be converted into corresponding sinusoidal voltage signals by using a plurality of matched current sources to drive each MR element independently and then using operational amplifiers (OpAmps) to derive the three sinusoidal output voltages. Alternatively, the resistance changes of the MR elements can be converted into corresponding sinusoidal voltage signals by using a constant voltage source to drive the plurality of MR elements utilizing voltage dividers and OpAmps to derive the sinusoidal output voltages.
In a second implementation of the present invention, the single MR sensor consists of an MR die wherein each of a plurality of MR elements consists of three interdigitated electrically isolated MR segments collectively forming a circular area, wherein OpAmps are obviated and the sensor circuit is totally passive. This permits one to use electrically independent, but magnetically interdependent voltage dividers to generate three sinusoidal signals with 120 degree phase shifts.
Structurally, in the second implementation, the present invention is composed of an MR die wherein each MR element of a plurality of MR elements thereof consists of three interdigitated electrically isolated MR segments wherein each MR segment is characterized by a magnetosensitive material. Respective interdigitated MR segments of each of the plurality of MR elements are electrically connected such as to form an MR sensor consisting of three groups of a plurality of interdigitated segments apiece. In the second implementation of the present invention, the MR segments are composed of indium antimonide (InSb) epitaxial film mesas, or film mesas of another suitable magnetoresistive material. The ends of the MR segments of each MR element are connected to their respective bonding pads (or terminals) by which electrical connections may be made to the MR die. The techniques to fabricate the MR elements are elaborated in the aforementioned U.S. Pat. Nos. 5,153,557 and 5,184,106.
It is preferable that the respective corresponding MR interdigitated segments within each group be matched to each other and that the geometry of the interdigitation of the MR segments is such that the magnitude of the increase of the resistance of one MR segment within one group is the same as the magnitude of the decrease in resistance of a respective corresponding MR segment within a diametrically opposed group, but this is not essential. Proper circuit design with appropriate weighting factors determined empirically or theoretically can be applied by those of ordinary skill in the art to accommodate MR segment mismatch and interdigitation geometries.
The MR die is mounted on a bias magnet and affixed to face toward the end face of the motor shaft, wherein at least an end portion thereof is composed of a ferromagnetic material which provides a magnetic irregularity. The center of the shaft is aligned with the center of the die and the end face of the shaft is parallel to the plane of the MR die. One half of the end face is a few millimeters shorter than the other half creating a half circular tooth covering one half of the total MR die area. When the motor rotates, that tooth sweeps through the MR segments and covers one half of the total MR die area at all times. Due to the higher magnetic flux density under the tooth, the MR segments under the tooth increase their resistance resulting in a resistance modulation of each MR segment between a maximum resistance and a minimum resistance. By a proper selection of the MR groups one can obtain a plurality of periodically varying voltage signals having a predetermined phase spacing therebetween, as for example three sinusoidal voltage signals spaced 120 degrees apart. For example, the resistance changes of the MR groups can be converted into corresponding sinusoidal voltage signals by using a constant voltage source to drive the MR segments and then using three voltage dividers thereby enabling the MR sensor to be totally passive.
Accordingly, it is a preferred object of the present invention to provide a position sensor wherein a single MR die consisting of a plurality of matched pie-shaped MR elements, collectively forming a circular area, acts as a single MR sensor to sense rotor position from the position of a ferromagnetic tooth on a shaft of a motor with respect to the MR sensor.
It is an additional preferred object of the present invention to provide a position sensor wherein a single MR die has a plurality of MR elements consisting of three interdigitated electrically isolated MR segments such that each MR segment is characterized by a magnetosensitive material, wherein the MR die acts as a single MR sensor to sense rotor position from the position of a ferromagnetic tooth on a shaft of a motor with respect to the MR sensor.
These, and additional objects, advantages, features, and benefits of the present invention will become apparent from the following specification.